Open Fractures in Jurassic Sandstone Reservoirs in the SW-Barents Sea, Some Implications for Fluid Flow and Reservoir Performance

Ole Petter Wennberg¹, Lars Rennan², Frode Karlsen³, and Remy Basquet^1
1Statoil ASA, Stavanger, Norway
²Statoil ASA, Trondheim, Norway
³Statoil ASA, N-9401 Harstad, Norway

Reservoir properties in sandstone reservoirs on the Norwegian continental shelf are generally assumed to be controlled by depositional environment and diagenesis during subsequent burial. However, open fractures, when present, have a significant influence on reservoir flow. In fact, natural open fractures are observed in sandstone reservoirs of the Jurassic Tubåen, Nordmela and Stø Formation in the Snøhvit Field, and their geometry and impact on fluid flow have been studied in detail using CT-scanning techniques on core samples. Two types of fractures are observed: F1 fractures are short and stylolite related, and F2 fractures are longer crosscutting the core. The fractures are steep and dominantly striking N-S with a significant spread. Although, the F1 fractures are short, a percolating and 3D connected open fracture network across the core was found in most samples. The formation of the open fractures in this area is suggested to be related to thermoelastic processes during exhumation in late Tertiary time in a weakly anisotropic stress field. The matrix sandstone of the Snøhvit Field has properties of good quality, and open fractures are not necessary for an effective hydrocarbon production. However, flow experiments of a core monitored by CT-scanning indicate that the open fractures affect the fluid flow in parts of the reservoir succession. 1-phase miscible flooding experiment show approximately 3 times higher flooding velocity in an open F2 fracture compared to the matrix. 2-phase flooding by gas injection into a 100% water saturated core gave early gas breakthrough due to flow in the fracture and thereafter very little water production. Hence, the effect of open fractures should be considered when evaluating reservoir performance in the SW Barents Sea.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece